The relationships between marine growth and survival, and smolt sizes were examined for white-spotted charr (Salvelinus leucomaenis) populations in the Nairo River, Rebun Island, off northern Hokkaido Island, and the Haraki River, southern Hokkaido Island, Japan. Smolts in the Haraki River were much larger in size than those in the Nairo River, despite being similar size among age cohorts and between the sexes in each population. From scale analyses, smolt size-specific growth rates and survival were estimated by utilizing both observed and back-calculated size-frequencies. Inverse correlations between specific growth rates at sea and smolt size were found in both populations. However, the expected size increments at a given size of smolts in the Haraki River were significantly larger than of those in the Nairo River. For both populations, there were no significant effects of smolt ages (2 to 5 or 6 years) on growth rates at sea. Larger smolts had a consistent survival advantage, although the degree of size-dependent survival function seemed to differ between populations. The results suggested that growth and survival of individuals at sea are determined ultimately by size-, rather than age-, dependent factors depending upon local environmental conditions, supporting the hypothesis of a threshold size for smolting within populations.